Coextruded structures containing two or more different layers are becoming more commonplace, especially in the food packaging industry. Multi-layer plastic structures are displacing many materials, such as glass and metal containers, due to cost and performance factors. One route frequently used in the production of plastic containers is thermoforming from coextruded sheet. A typical structure will have an oxygen barrier layer to extend the shelf life, outer or cap layers for moisture, taste and/or odor barrier purposes, and adhesive layers to bond the barrier layer to the outer layers.
Laminates containing ethylene vinyl alcohol copolymer (EVOH) layers as the oxygen barrier layer are well known and are commonly used. EVOH is an excellent material for such use but it does have some drawbacks. For instance, the processability of EVOH in some thermoforming processes leaves something to be desired and it is difficult to make some types of containers, such as deep draw containers wherein the length/diameter ratio is greater than 1.0. The most common processing problem is fibrillation of the laminate which is the necking or thinning of the barrier layer in striations.
The processability of EVOH has been increased by blending it with various crystalline nylon materials, i.e., nylons which have a defined melting point. Such nylon materials generally have poor oxygen barrier properties and thus blending them with EVOH decreases the oxygen barrier properties of the EVOH material making the blend less suitable for many applications such as in the food and medical industries. Furthermore, many crystalline nylons require the use of plasticizers to make them compatible with the EVOH and these plasticizers may degrade the EVOH over a period of time.
Amorphous nylon materials having no melting point, have better oxygen barrier properties and exhibit better processability than the crystalline nylon materials discussed above. Such amorphous nylon can be used at lower temperatures so the gelling reaction which often occurs between nylon and EVOH can be prevented. Also, the oxygen barrier properties of amorphous nylon materials tend to increase as the moisture level goes up which can help compensate for the decrease in oxygen barrier properties of the EVOH as moisture level goes up.